Deduct 3 points from each question, and now he gets 85 points. Obviously, the single digit in his score should be 5. If it is a multiple of 3, now he will deduct 3 points for every wrong question, and 3+3+3 will get 15, and five questions will be wrong. If all the questions are correct, it will leave 10, minus 100.

So the answer is 10.